1. Field of the Invention
The present invention relates generally to a fluid bearing device for information equipments, audio and video equipments, business machines. More particularly, the invention relates to a fluid bearing device suitable for magnetic hard disk drive (HDD), fan motor and so forth to be used in a notebook type personal computer or the like.
2. Description of the Related Art
As a typical conventional fluid bearing device of the type set forth above is a spindle motor for HDD, for example. A construction of the spindle motor will be discussed with reference to FIG. 3 which is a section showing a construction of a spindle motor as the fourth embodiment of the present invention.
In the spindle motor, a cylindrical portion 101a is vertically extended from a base 101. On the cylindrical portion 101a, a sleeve 102 is fixed. A shaft 103 is rotatably inserted into the sleeve 102. On the upper end of the shaft 103, a reversed cup-shaped hub 104 is integrally mounted. Between the shaft 103 and the sleeve 102, a dynamic pressure fluid bearing portion is interposed.
Namely, on the lower end of the shaft 103, a disk shaped thrust plate 105 is secured by press fitting. Both planar surfaces of the thrust plate 105 serves as thrust receiving surface 105s of a thrust fluid bearing S. To the thrust receiving surface 105s on the upper surface side, a lower end surface of the sleeve 102 as a counter part member is placed in opposition. The lower end surface of the sleeve 102 serves as the thrust bearing surface 102s of the thrust fluid bearing S.
On the other hand, below the thrust plate 105, a counter plate 106 as another counter part member is arranged. The counter plate 106 is fixed to the base 101. The upper surface of the counter plate 106 is placed in opposition to the thrust receiving surface 105s on the lower surface side of the thrust plate 105 to form thrust bearing surface 106s of the thrust fluid bearing S. At least one of the thrust receiving surfaces 105s and the thrust bearing surfaces 102s and 106s, a thrust fluid bearing S having a not shown herringbone type or spiral type groove for generating a dynamic pressure, is constructed.
Furthermore, on the outer peripheral surface of the shaft 103, a pair of radial receiving surface 103r is formed. In opposition to the radial receiving surface 103r, a radial bearing surface 102r is formed on the inner peripheral surface of the sleeve 102. At least one of the radial receiving surface 103r and the radial receiving surface 102r has a herringbone type groove 107 for generating dynamic pressure for example, to form a radial fluid bearing R.
On the outer periphery of the cylindrical portion 101a, a stator 108 is fixed. The stator 108 opposes with a rotor magnet 109 fixed on the lower side of the inner peripheral surface of the hub 104 over the entire circumference to form a drive motor M for driving the shaft 103 and the hub 104 for rotation in integral manner.
When the shaft 103 is driven to rotate, by pumping action of respective grooves for generating dynamic pressure of the thrust fluid bearing S and the radial fluid bearing R, dynamic pressure is generated in lubricant in bearing clearances of the fluid bearings S and R. The shaft 103 is supported in non-contact manner with the sleeve 102 and the counter plate 106.
Such conventional spindle motor is constructed with a stainless steel having high Young""s modules (Vickers hardness Hv=about 270) for certainly obtaining a joint strength by press fitting of the thrust plate 105 and the shaft 103 to assure impact resistance against external shock. The sleeve 102 and the counter plate 106 as counterpart member is constructed with a copper alloy of the same composition (e.g. free cutting brass of Vickers hardness Hv=about 150). On the other hand, the groove for generating dynamic pressure of the thrust fluid bearing S is processed by etching on both planar surfaces of the thrust plate 105.
In the recent spindle motor for HDD, it has been required superior durability in starting and stopping for assuring reliability for a long period. Particularly, in case of dynamic pressure fluid bearing, it is inherent to cause mutual contact between the thrust bearing surface 106s and the thrust receiving surface 105s upon starting and stopping. Therefore, repeating of starting and stopping inherently cause wearing to increase wearing tip which can be bit in the bearing to degrade precision of rotation or in the worst case to cause failure of rotation.
Accordingly, it is important to prevent the thrust bearing surface 106s and the thrust receiving surface 105s from being damaged due to contact upon starting and stopping.
However, in the conventional thrust fluid bearing S, since the groove for generating dynamic pressure is in the thrust plate 105 which is formed with the stainless steel having high hardness, fine burr or bulge portion around the peripheral portion of the groove to be created during etching process, cannot be removed completely. Therefore, by repeating of starting and stopping, it is possible to damage the bearing surface (thrust bearing surface 102s and the thrust bearing surface 106s) of the counterpart member (sleeve 102 and counter plate 106) formed with copper alloy having low hardness.
On the other hand, since the stainless steel is not good in cutting ability, difficulty is encountered in assuring dimensional precision to cause manufacturing ability.
Therefore, the present invention is to provide a fluid bearing device solving the problem in the conventional fluid bearing and superior in wear start-stop resistance and in manufacturing ability.
In order to accomplish the above-mentioned object, a fluid bearing device comprises:
a shaft having a flange portion;
a sleeve opposing to the shaft across a fluid bearing clearance of a radial fluid bearing;
a counterpart member opposing to at least one of plane of the flange portion across a fluid bearing clearance of a thrust bearing,
the flange portion and the sleeve portion being formed of copper alloy of mutually difference composition.
With the construction set forth above, the fluid bearing device according to the present invention facilitates certainly obtaining dimensional precision, achieves superior workability and mass-productivity since the flange portion and the sleeve are formed of copper alloy having high cutting ability.
Since the flange portion and the sleeve are formed with the copper alloys mutually having different compositions, high workability in formation of the dynamic pressure generating groove can be achieved for accomplishing high mass-production ability by machining the dynamic pressure generating groove on one of the flange portion and the sleeve having lower hardness. In addition, fine burr or bulged portion around the groove formed during machining of the groove, can be completely removed for successfully preventing damaging of the bearing surfaces of the member, to which the flange portion or the sleeve contact due to repeated staring and stopping of the bearing.
It is preferred that the composition of the copper alloys forming the flange portion and the sleeve are selected to that the difference of hardness in Vickers hardness Hv is greater than or equal to 50. Thus, the bearing surface of the member is hardly damaged to achieve high durability in starting and stopping. When the hardness of the flange portion and the sleeve is the same, the dynamic pressure generating groove may be formed in either one of or both of the flange portion and the sleeve.
On the other hand, in the fluid bearing device, a copper alloy forming the sleeve is a copper alloy having Vickers hardness Hv 180 or higher, and more preferably having Vickers hardness Hv 200 or higher.
Also, in the fluid bearing device of the present invention, the copper alloy can be any one of beryllium copper, high strength brass and aluminum bronze.
The copper alloy having Vickers hardness Hv 180 or higher (more preferably having Vickers hardness Hv 200 or higher) has high workability (cutting ability, plastic working or so forth) with superior sliding ability, superior in mass-production at low cost, and superior in durability in starting and stopping.
It should be noted that the copper alloy forming the sleeve is selected to the Vickers hardness Hv greater than or equal to 300 to significantly enhance durability in starting and stopping of fluid bearing device.
Furthermore, by taking beryllium copper as the copper alloy and providing age hardening process to provide Vickers hardness greater than or equal to 350, durability in starting and stopping of the fluid bearing device can be further enhanced. Beryllium has Vickers hardness Hv about 210 to 270 even before age hardening process. In this case, after forming the dynamic pressure generating groove by plastic working, such as ball rolling process, and then performs the age hardening process for providing Vickers hardness Hv greater than or equal to 350, durability in starting and stopping can be enhanced without degrading workability and mass-production environment.
The counterpart member may be formed of copper alloy.
A dynamic pressure generating groove of a depth in a range of 2 to 10 xcexcm forming the radial fluid bearing, may be provided on an inner periphery of the sleeve. With the construction set forth above, plastic working, such as ball rolling process process, becomes easier. Of course, the groove may be machined not only by ball rolling process but also by cutting process or other methods.
For example, copper alloy having Vickers hardness Hv greater than or equal to 180 (more preferably the copper alloy having Vickers hardness Hv is 200) has higher hardness than free cutting brass (Vickers hardness Hv is about 150) to make processing difficult. Accordingly, when the depth of the groove exceeds 10 xcexcm, load becomes significant if the groove is formed by plastic working, such as ball rolling process or the like to easily cause failure to make mass-production difficult. If the depth of the groove is less than 2 xcexcm, the dynamic pressure generate by pumping action of the groove associating with rotation of the bearing becomes too small to obtain predetermined bearing performance. By setting the depth of the groove in the range of 2 to 10 xcexcm, both of workability and bearing performance can be achieved. For further facilitating processing of the groove, it is further preferred to set the depth of the groove within a range of 2 to 6 xcexcm.
It should be noted that when the copper alloy is beryllium copper, in the similar reason set forth above, the depth of the groove is to be set within a range of 2 to 8 xcexcm, and more preferably in a range of 2 to 6 xcexcm.
Furthermore, the fluid bearing device according to the present invention, the flange portion may be fixed to the shaft by threading. With such construction, the push-out force of the flange portion will not be restricted by longitudinal elastic modulus as in the case where the flange portion is press fitted. Therefore, sufficiently high impact resistance can be certainly provided.
Furthermore, the sleeve and the counterpart member may be mutually different in material or hardness.
Furthermore, a plane of the sleeve opposes one of planes of the flange portion across a fluid bearing clearance of the thrust bearing, a plane of the counterpart member opposes another plane of the counterpart member across a fluid bearing clearance of the thrust bearing, and at least one of the plane of the sleeve and the plane of the counterpart member is provided surface treatment.
By this, in comparison with that both of the sleeve and the counterpart member are formed with the material having high hardness and high sliding ability or by providing surface process to achieve higher hardness and higher sliding ability, substantially comparable durability in starting and stopping can be achieved with lower cost.
A fluid bearing clearance of the radial fluid bearing and a fluid bearing clearance of the thrust fluid bearing are filled with lubricant containing 0.1 to 5.0 Wt % of antioxidant.
With such construction, reaction between the lubricant and copper alloy can be restricted to make it possible to reduce reduction amount of the lubricant due to evaporation at high temperature. If the content of the antioxidant less than 0.1 Wt %, the foregoing effect becomes insufficient. If the content of the antioxidant exceeds 5 Wt %, viscosity of the lubricant becomes different from that of the base oil.